Electrophotographic printing is well known and has been widely refined. For example, today, almost every office and indeed some homes have electrophotographic copiers. The industry has grown to the point where it is now a highly competitive multi-billion dollar industry. In most instances, these home and office copiers are capable of providing only about a few copies per minute.
In electrophotography, images are photoelectrically formed on a photoconductive layer mounted on a conductive base. Liquid or dry developer or toner mixtures may be used to develop the requisite image.
Liquid toner dispersions for use in the process are formed by dispersing dyes or pigments and natural or synthetic resin materials in a highly insulating, low dielectric constant carrier liquid. Charge control agents are added to the liquid toner dispersions to aid in charging the pigment and dye particles to the requisite polarity for proper image formation on the desired substrate.
The photoconductive layer is sensitized by electrical charging whereby electrical charges are uniformly distributed over the surface. The background area of the photoconductive layer is then exposed by projecting or alternatively by writing over the surface thereof with a laser, L.E.D., or the like. The electrical charges on the photoconductive layer are conducted away from the areas exposed to light with an electrostatic charge remaining in the image area. The charged pigment and/or dye particles from the liquid toner dispersion contact and adhere to the image areas of the photoconductive layer. The image is then transferred to the desired substrate, such as a travelling web of paper or the like.
In contrast to office and home copiers, high speed electrophotographic printing presses are being developed wherein successive images are rapidly formed on the photoconductive medium for rapid transfer to carrier sheets or the like travelling at speeds of greater than 100 ft./min. and even at speeds of from 300-500 ft./min. As can be readily understood, such high speed machines readily consume the solid pigment and/or dye and associated resin particles from the liquid toner baths.
As noted above, after the requisite image has been formed on the electrophotoconductive surface by the attraction of the color-imparting solids toner particles to the image portions of the latent electrostatic image, it is necessary to efficiently transfer that formed image to the desired substrate such as a travelling web of paper or similar article.
In order to prevent smearing and distortion of the image during the transfer process, it is necessary to ensure that the speed of the travelling web is precisely synchronized with the peripheral speed of the rotating electrophotoconductive cylinder during this transfer process. Moreover, so as to provide for high print quality character definition, it is necessary that the transfer process result in effective transfer of almost all of the color-imparting toner solids particles from the rotating electrophotoconductive surface to the travelling web.
These and other objects are met by the invention hereof that provides for efficient high speed transfer of the liquid toner dispersion travelling on the rotating electrophotoconductive cylinder to an adjacent, travelling web of paper or the like.